New putative virulence factors of Streptococcus suis involved in invasion of porcine brain microvascular endothelial cells.

Streptococcus suis serotype 2 is an important pathogen causing a wide range of infections in swine, the most important being meningitis. Few virulence factors have been identified and the pathogenesis of infection is not well understood. Recently, we demonstrated the ability of S. suis to adhere to and invade porcine brain microvascular endothelial cells (PBMEC) forming the blood-brain barrier. In this paper we describe the screening of a mutant library, produced by insertion of transposon Tn917 into the chromosome of S. suis strain P1/7, for mutants that are less able to interact with PBMEC. Both qualitative and quantitative screening assays were used to identify poorly invasive mutants. Tn917 insertion sites from nineteen poorly invasive mutants were sequenced and characterized. Five mutants were selected and their virulence was assessed in a mouse model of infection. Two out of these five mutants were attenuated as measured by decreased colonization of organs, as well as reduced mortality and morbidity. When tested in swine these two attenuated mutants led to decreased bacterial loads in blood, less severe and delayed clinical signs, and lower plasma IL-6 levels than did infection with the wild-type strain. Overall, our results suggest that these two genes may contribute to the virulence of S. suis.